A conventional inverter comprises two switches S1 and S2, two power supplies E1 and E2 and a LC series circuit consisting of reactor L1 and capacitor C2 which is connected between a junction point of the two switches S1 and S2 and a junction point of the two power supplies E1 and E2 as is indicated in FIG. 2. When the switch S1 is on and the switch S2 is off, current iL flows in a direction indicated by the arrow through the LC series circuit. On the contrary, when the switch S1 is off and the switch S2 is on, the current iL flows in an opposite direction through the LC series circuit.
By turning on and off the switches S1 and S2 alternately, the direction of the current flowing through the LC series circuit can be continuously changed. Thus, when the switches are turned on and off at a speed T=1/Fo which is approximate to an intrinsic resonance frequency (see the following Expression 1) of the LC series circuit, a voltage VL1 (see the following Expression 2) is generated across the reactor L1 while voltage VC1 (see the following Expression 2) is generated across the capacitor C1. EQU Fo=1/2.pi..sqroot.LC (Expression 1) EQU VL1=Ldi/dt, VC1=1/C.times..intg.idt (Expression 2)
FIG. 1 shows a circuit of a discharge lamp operating device employing a self-excited inverter, to which the above principle is applied, reconstructing the circuit in FIG. 2. The circuit in FIG. 1 is provided with semiconductor devices, that is transistors Q1 and Q2, for use in place of switches S1 and S2. Instead of the power supplies E1 and E2 of the circuit in FIG. 2, the circuit in FIG. 1 has an operating power supply E for supplying power from the outside, and capacitors C2 and C3 for storing power are connected to perform the same function as the power supplies E1 and E2 respectively. Thus, the circuit in FIG. 1 is configured to be equivalent to the circuit in FIG. 2. In order to turn on and off the transistors Q1 and Q2 alternately, an oscillation transformer T1 is inserted between a junction point of the transistors Q1 and Q2 and the reactor L1, and secondary side coils of the oscillation transformer T1 are connected between a base and an emitter of the transistors Q1 and Q2, respectively, in such a way that directions of induction of voltages in the secondary side coils oppose each other.
When an actuating signal is supplied to the transistor Q2 in FIG. 1, the transistor Q2 is turned on and a current iL starts flowing in a direction opposite to that indicated by the arrow. If a voltage induced to the secondary side of the oscillation transformer T1 turns off the transistor Q1 and sufficiently turns on the transistor Q2 and the oscillation transformer T1 becomes saturated, then the directions of induction of the voltages in the secondary side coils of the transformer TI are reversed. By turning on the transistor Q1 and turning off the transistor Q2, the current iL starts flowing in a direction indicated by the arrow in FIG. 1. When the oscillation transformer T1 becomes saturated, the directions of induction of the voltages in the secondary side coils of the oscillation transformer T1 are reversed and then the transistor Q1 is turned off and the transistor Q2 is turned on. This operation is repeated in a self-excitatory (self-excited) manner without supplying any external signals, at which time a voltage represented by the following expression 3 is generated across capacitor C1. EQU VC1=1/C .times..intg.idt (Expression 3)
In the circuit described in FIG. 1, a hot-cathode discharge lamp LA is connected across the capacitor C1 so that a voltage generated across the capacitor C1 is transferred to the hot-cathode discharge lamp LA to operate the hot-cathode discharge lamp LA. The configuration of the circuit in FIG. 1 is common to the conventional hot-cathode discharge lamp operating devices employing a self-excitatory inverter.
In a hot-cathode discharge lamp operating device employing a conventional self-excitatory inverter, all of the current running from the LC series resonance circuit to the capacitor flows through filaments on both sides of the hot-cathode discharge lamp and, therefore, a filament heating voltage Vf is represented as Rf.times.iL provided that the filament's internal resistance is Rf. Thus, as the filament heating voltage Vf varies according to the current running through the capacitor of the LC series resonance circuit, the filament heating voltage Vf cannot be appropriately adjusted and as a result, thermal electrons are emitted through only one or two points within the hot-cathode discharge lamp, where intense heat is produced. Thus, shortening the lifetime of the filaments.
Further, according to the prior art, when a supply voltage varies, the output frequency also varies and a scope of change in high-frequency output expands and, thereby, the voltage across the capacitor C1 of the LC resonance circuit changes, which changes the illuminance of the lamp. Therefore, it is difficult to supply the preheat voltage to the filament at an initial stage of lighting the lamp. It is also difficult to construct a control circuit for dealing with the terminal phenomenon of the hot-cathode discharge lamp. Thus, the operating efficiency of the hot-cathode discharge lamp deteriorates, and the reliability of a discharge lamp operating device is compromised.
Given the above, it is the object of the present invention to obviate the aforementioned problems of the prior art and to provide a discharge lamp operating electronic device which enables a prolonged service life of the hot-cathode discharge lamp and provides improved reliability of the operating device.